AUSTRALIAN MUSEUM MEMOIR 18 Papers from the Conference on the Biology and Evolution of Crustacea HELD AT THE AUSTRALIAN MUSEUM SYDNEY, 1980 Edited by JAMES K. LOWRY The Australian Museum, Sydney Published by order of the Trustees of the Australian Museum Sydney, New South Wales, Australia 1983 Manuscripts accepted for publication 1 April, 1982 7 CA TOMERUS POLYMERUS AND THE EVOLUTION OF THE BALANOMORPH FORM IN BARNACLES (CIRRIPEDIA) D.T.ANDERSON School of Biological Sciences, University of Sydney Sydney, N.S.W. 2006, Australia SUMMARY On the basis of comparative anatomy, Darwin proposed that the catophragmid C. polymerus, representing the basic form among balanomorphs, had evolved from a scalpellid lepadomorph ancestry. This hypothesis has since been supported by fossil evidence, but has not been tested by the techniques of comparative functional morphology. Functional studies of C. po!ymerus and Ca!antica villosa have now shown that an evolution of catophragmid balanomorphs from calanticine scalpellids is functionally feasible, upholding Darwin's proposal. C. polymerus retains the scalpellid mode of planktivorous extension feeding, except for further modification of the first two pairs of cirri as short maxillipeds acting in forward food transfer in a limited space. The basic adaptive significance of the foreshortened form and flattened operculum of C. po!ymerus lies in allowing a species with this mode of feeding to inhabit a high energy intertidal environment. Protection against certain kinds of predation may also have been important. The development of the opercular valves from capitular plates and their changed orientation relative to the wall is accommodated by further modification of a hinge mechanism already present in calanticine scalpeIlids. The closure mechanism of the operculum involves supplementation of the action of the adductor scutorum by the downward pull of large tergal depressor muscles, evolved as a modification of the peduncular longitudinal muscles of calanticines. Massive tergal and small scutal depressor muscles, a basic balanomorph condition, are functionally associated with a large prosoma and paired branchiae occupying the rostral part of the limited mantle cavity. Introduction Catomerus polymerus (Darwin) is a common balanomorph barnacle on the rocky shores of southeastern Australia (Pope, 1965), easily distinguished by its eight wall plates supplemented by several concentric whorls of marginal plates (Fig. la). The recent revision of the Balanomorpha by Newman and Ross (1976) follows Utinomi (1968) and places C. po!ymerus in the family Catophragmidae of the superfamiIy Chthamaloidea. The species was first described by Darwin (1854) in his monograph on the Balanidae, from specimens collected from Twofold Bay, N.S.W. Darwin placed his specimens in the genus Catophragmus, previously erected by Sower by (1826) for a similar animal, Catophragmus imbricatus Sowerby, known as two shells from Antigua in the West Indies in the collections of the British Museum. Pilsbry (1916) with more material of C. imbricatus at his disposal from Bermuda, recognised a difference between the two species sufficient to require transfer of C. polymerus to another genus, which he named Catomerus. A difference already noted by Darwin, is the presence in C. imbricatus of a pair of caudal appendages, absent in C. polymerus. The distinction between the two is otherwise minor, comprising slight diff~rences in the operculum and basis. In a personal communication, W.A. Newman writes "I have despaired of finding much if any differences in the appendages between the two." At the same time, it has been recognised recently that C. imbricatus is now confined to the tropical W. Atlantic (Antigua and Bermuda), while the animals on the other side of the isthmus, in Panama and Costa Rica, are a distinct species, C. pilsbryi Broch (Southward and Newman, 1977). The Catophragmidae also includes another surviving form, Chionelasmus darwini (Pilsbry), which apparently has a disjunct distribution, Hawaii and New Zealand in the Pacific Ocean and the Rodriguez 8 D.T. ANDERSON Fig. I. a, Catomerus po!ymerus, apical view. b, Pollicipes po!ymerus, right lateral view. e, carina; cl, carinolateral; I, lateral; p, peduncle; r, rostrum; rt, rostrolateral; s, subcarina; se, scutum; sr, subrostrum; le, tergum. Islands in the Western Indian Ocean, as a benthic shelf form at 450-460 m (Stanley and Newman, 1980). Chionelasmus is more advanced than Catophragmus or Catomerus in some features of its carapace. The wall plates are 6 in number, as in most balanomorphs, and there is only a single whorl of supplementary plates outside the primary wall plates. The general anatomy of the body and limbs, however, remains catophragmid. Catomerus polymerus is therefore a typical representative of the Catophragmidae. Darwin, in a delightful example of his thinking in evolutionary terms, recognised immediately the phylogenetic importance of this animal, which "forms in a very remarkable way the transitional link between" the lepadomorphs and the balanomorphs, "for it is impossible not to be struck with the resemblance of its shell with the capitulum of Pollicipes" (see Fig. 1b). "In Pollicipes, at least in certain species, the scuta and terga are articulated together-the carina, rostrum and three pairs of latera, making altogether eight inner valves, are considerably larger than those of the outer whorls-the arrangement of the latter, their manner of growth and union,-all are as in Catophragmus. If we, in imagination, unite some of the characters found in the different species of Pollicipes, and then make the peduncle so short (and it sometimes is very short in P. mite/la) that the valves of the capitulum should touch the surface of attachment, it would be impossible to point out a single external character by which the two genera in these two distinct families could be distinguished." While Darwin went on to recognise that there were some fairly major internal differences, especially in the arrangement of the mantle musculature, that he was unable to explain, he saw Catomerus not only as an indicator of a scalpellid ancestry for the baianomorph form, but also as a form ancestral to other baianomorphs. "Considering the whole structure, external and internal, of Catophragmus, with the great exception of the exterior whorls of valves, there is hardly a single generic character by which it can be separated from Octomeris and Pachylasma"; and the generalised position of Octomeris and Pachylasma at the base of the balanomorph stock continues to be recognised to this day (Newman and Ross, 1976). Darwin, then, presented a hypothesis of the origin of the Balanomorpha-that they had evolved from a scaipellid ancestry via forms represented today by the catophragmids, which in turn gave origin to the other balanomorphs. EVOLUTION OF BALANOMORPH FORM IN BARNACLES 9 se te Fig. 2. a, Seillae/epas dorsata, right lateral view. b,S. dorsata, plan of capitular plates. c, Catomerus polymerus, right lateral view. 11, C. po/ymerus, plan of wall plates. (From Newman, Zullo and Withers, 1969, reproduced with permission, courtesy of the Geological Society of America and University of Kansas.) e, carina; cl, carinolateral; I, lateral; r, rostrum; rI, rostrolateral; se, scutum; te, tergum. In the years since 1854 this hypothesis, based on comparative anatomy, has gained further support from fossil evidence. Withers (1935) described from the Cretaceous of Sweden a fossil catophragmid, Pachydiadema cretaceum, which remains the only known Mesozoic balanomorph. This discovery placed the catophragmids chronologically at the base of balanomorph evolution in the later Mesozoic, a time of abundant and diverse sca\pellids (Newman, Zullo and Withers, 1969). P. cretaceum, furthermore, shows opercular features more like those of scalpellids than the modern catophragmids, which can now be recognised as relicts of a family of once widespread distribution (Stanley and Newman, 1980). Newman et al. (1969) following Aurivillius (1894), developed the idea of a scalpellid ancestry further in a comparison (see Fig. 2) of the plate arrangements of C. polymerus and the Eocene calanticine Scillaelepas dorsata (Steenstrup). Allowing for only a minor modification of the position of the laterals in relation to the rostrolaterals and carinolaterais, the arrangement of the capitular plates in C. dorsata is one which can be easily envisaged, with foreshortening of the stalk, to have become modified to that of the wall plates of C. polymerus. Since Scillaelepas and Calantica are thought to be central and early evolved genera within the calanticine scalpellids, first appearing in the Upper Jurassic (Newman, 1979), Darwin's hypothesis gains further support from this evidence, as it does from the cirral and mouth part anatomy of the two groups. Pollicipoid scalpellids, including calanticines and pollicipines (Zevina, 1978), show some modification of the first and to a lesser extent the second pair of cirri as maxillipeds (Figs 3a, 3d), and have a characteristic setation of the four posterior pairs of cirri (Fig. 4b). Each cirral segment carries several pairs of long setae anterolaterally and has a bunch of short, forwardly projecting setae between them on the anterior face of the segment. C. polymerus shares this feature in the setation of the four posterior pairs of cirri (Fig. 4a), but it exhibits a greater modification of the first two pairs (Figs 3b, 3c) as short maxillipeds (Darwin, 1854; Pope, 1965). A striking similarity also attends the 10 D.T. ANDERSON a c d Fig. 3. a, Left cirrus I of Calantica villosa, median view. b, Left cirrus I of Catomerus polymerus, median view. c, Left cirrus II of C. polymerus, median view. d, Left cirrus II of C. villosa, median view. en, endopod; ex, exopod. oral cone and mouthparts (Fig. 5) (Darwin, 1854; Pilsbry, 1916; Nilsson-Cantell, 1926; Pope, 1965; Newman and Ross, 1976; Foster, 1978). The labrum is bullate. The palps are simple, and setose mainly at the tips. The mandibles are tri- or quadridentoid, with a well developed incisor tooth and a strong, setose or pectinated molar process.